1. Field of the Invention
The present invention relates to an ignition distributor for an internal combustion engine and, more specifically to an ignition distributor rotor designed to suppress noise electric wave derived from a discharge between a rotor electrode and each of cap electrodes.
2. Description of the Prior Art
Various studies have shown that one of the sources of motor vehicle noise electric wave radiation is the breakdown of the arc gap between a discharge face of the ignition distributor rotor electrode and each of the circumferentially disposed distributor cap electrodes. The arc gap is generally termed the "distributor gap" and hereinafter will be so referred to.
In an ignition system for a spark ignition internal combustion engine, a large current flows with a steep rise time upon spark discharge within an ignition distributor. The radiation of noise electric wave due to the large current flow disturbs radio broadcasting service, television broadcasting service and other kinds of radio communication systems. Further, noise causes operational errors in electronic control circuits of vehicle control systems, for example, E.F.I. (electronic controlled fuel injection system), E.S.C. (electronic controlled skid control system) or E.A.T. (electronic controlled automatic transmission system), and as a result, traffic safety is threatened. Thus, it is demanded to suppress radiation of the noise from the ignition system.
In operation of the ignition system, a high electric voltage supplied from the ignition coil rises toward its peak not in a step-like manner, but with a time constant determined by the circuit constant. When the voltage rises to a value high enough to cause the breakdown of the distributor gap, the gap is broken down to allow a discharge. In this case, when the level of voltage rises high enough to cause the breakdown of the distributor gap, a discharge current flows rapidly within a short pulse width. The discharge current has a high peak value and very unstable, so that a great deal of toxic high frequency components are generated and noise wave radiates using high tension cables as an antenna.
It is recognized that noise electric field radiating from a source of noise is proportional to a capacity discharge current. Therefore, for the purpose of suppressing noise electric wave, it is necessary to reduce capacity discharge current flowing across the distributor gap. The capacity discharge current occurs when the electric charges accumulated with floating capacity between the rotor electrode and cap electrodes move upon the breakdown of the distributor gap. The capacity discharge current has a high peak value and a steep rising time.
There have been proposed the following countermeasures against the noise radiation from an ignition distributor.
(A) An ignition distributor using a rotor electrode with a resistor:
The rotor electrode having a resistor embedded thereto is called as a "resistor rotor." Because there is a distributed capacity in parallel to the resistor of the distributor rotor, noise radiation within a high frequency band which exceeds 300 MHz is not reduced to a satisfactory level. As the resistor rotor has a resistance of the order of several k ohm, a loss in ignition energy is considerably great.
(B) An ignition distributor wherein the distributor gap between a rotor electrode and each of cap electrodes is wide:
The distributor gap measures from 1.524 mm to 6.35 mm. Although the ignition distributor with the widened distributor gap is effective in suppressing noise radiation, a loss in ignition energy is great. The great loss on the ignition energy is against the recent requirement on the ignition system that the ignition take place securely with a sufficient energy for the purpose of enhancing purification of exhaust gases and fuel economy.
(C) An ignition distributor with a third electrode:
The third electrode is attached to a rotor electrode via a dielectric therebetween. The breakdown of a gap between the third electrode and each of cap electrodes induces the breakdown of a gap between the rotor electrode and each of the cap electrodes. A disadvantage of this ignition distributor is in its complicated structure which causes a less reliable operation after a long use.